How many triangles can be inscribed in the heptagon pictured, where the three vertices of the triangle are also vertices of the heptagon?

A.9 B. 21 C. 35 D. 140 E. 210.

Ans: C. Grockit solves this by using combinations method and I agree with this solution. If the question mentions non-overlapping triangle, will the answer be 5. By using formula (n-2) *180, so here it will be 5 * 180 so 5 triangles, is this approach correct? And if the question was rephrased as number of triangles (does not mention non - overlapping) in a hexagon by the combination method answer will be 6 factorial / 3 factorial * 3 factorial. So 20 triangles, is this correct too? please confirm...